1. Field of the Invention
This invention pertains to flexible manufacturing systems, and more particularly to apparatus for processing selected workpieces into an assembled product.
2. Description of the Prior Art
Various equipment has been developed to transport individual components to a work station where selected operations are performed on them. For example, it is known to weld two or more pieces of preformed sheet metal into a sturdy automobile bodyside in an automatic assembly machine. The sheet metal workpieces are clamped into accurate relation to each other at a work station of the automatic assembly machine, and the workpieces are then welded together by robots or other processing equipment.
Transporting individual workpieces to and accurately locating them at a work station on a production basis presents several difficult problems. The clamps and other tooling at the work station present obstacles that must be avoided by the incoming and departing workpieces. The workpieces must be accurately positioned within the work station so that the tooling can properly clamp them together and the robot or other processing equipment can correctly perform the necessary operations. In addition, high production rates and reliability over a long service life are critical.
In some assembly applications, workpieces best approach the work stations from above. Accordingly, prior equipment has been developed that transports workpieces along an overhead monorail. Trolleys traveling on the monorail have electric motors that are powered via sliding contacts from electrical wiring carried on the monorail. The trolleys travel in a downstream direction above the various work stations an support the workpieces at a height sufficient to clear all the tooling at the work stations. To perform an operation on a workpiece, the trolley transporting it stops at a rail section located directly over the appropriate work station. A mechanism operates to lower the rail section and trolley such that the workpiece is properly positioned within the tooling at the work station. When the particular operation has been completed, the rail section, trolley, and workpiece are raised such that the lowerable rail section is again in line with the adjacent fixed upstream and downstream rail sections. Then the trolley is controlled to transport the workpiece downstream to be over the next work station, and the cycle is repeated.
Although the prior monorail systems have enjoyed some success, they nevertheless have important disadvantages. A primary drawback is that joints in the electrical conductors that power the trolleys must be provided between the lowerable rail sections and the adjacent fixed upstream and downstream rail sections. It is very difficult to consistently and reliably return the conductors of a lowered rail section into exact alignment with the corresponding wires of the adjacent fixed rail sections. Consequently, the conductors of a lowerable rail section occasionally mismatch with the corresponding conductors of the adjacent fixed rail sections, which results in faulty and unreliable system operation. In addition, great care must be exercised to be certain that a trolley and workpiece upstream of a lowered rail section do not travel downstream and fall off the adjacent upstream rail section on top of the lowered rail section.
A further disadvantage of prior monorail systems is that the relatively great weight of the lowerable rail section, trolley, and workpiece requires that the lowering and raising motions be performed at relatively slow speeds. As a result, cycle times are undesirably long, and productivity is correspondingly low.
It is known in prior monorail assembly systems to provide tooling for more than one product proximate a work station and to index the desired tooling to the work station as required. However, the prior systems are designed such that the tooling indexes in a direction transverse to the direction of motion of the workpieces to and from the work station. That design necessitates that the robots or other equipment be mounted on a structure that is tiltable on the factory floor to enable the robots to approach, process, and retract from the workpieces at the work station. Such construction is very expensive, and it limits accessibility to the tooling and processing equipment for maintenance and alterations.
Thus, a need exists for improvements in assembly machine workpiece handling systems.